This invention relates in general to tapered roller bearings and, more particularly, to a tapered roller bearing that is characterized by low wear and low power loss.
The gearing in an automotive differential must operate with considerable precision to keep noise and wear at a minimum. Typically, that gearing includes a ring gear and a pinion both of which rotate in a differential housing. The pinion meshes with ring gear to transfer torque from a drive shaft to the ring gear which in turn transfers the torque to axle shafts that extend from the housing. Both gears have spiral teeth, that is to say they are hypoid gears.
The pinion must rotate with considerable stability, that is to say, about a fixed axis and in a fixed axial position. To this end, the pinion is carried on a shaft that rotates in a pair of tapered roller bearings that are fitted to the differential housing where they are mounted in opposition and set to a condition of preload. While the arrangement gives the stability requiredxe2x80x94at least at the outsetxe2x80x94the tapered roller bearings in current use require a measure of torque to overcome friction within the bearings themselves as well as the churning of the lubricant within them. This torque consumes power. Moreover, the bearings in time experience wear, and this detracts from the stability, so that the pinion may migrate from the position that provides the most effective engagement with the ring gear.
Several factors affect the torque required by a tapered roller bearing and the wear that the bearing experiences. For example, small angles on the raceway cause the rollers to fit tightly between the raceways under the preload, thus increasing the friction in the bearing and wear as well. When the rollers are long in comparison to their width, the longer lines of contact with the raceways result in significant churning of the lubricant film which separates the rollers from the raceways, and this requires torque. Typically, the rollers are slightly crowned to prevent excessive edge-loading, but even so, between the ends the crowning diminishes considerably, and here the churning occurs along the lines of contact. The typical tapered roller bearing has a thrust rib at the large diameter end of the raceway on its inner race or cone, and the rib prevents the rollers from moving up the raceway and being expelled. Normally the height of the rib amounts to about 20% of the diameter of the rollers at their large ends. This concentrates the contact forces in relatively small areas, and accelerates wear. A rough surface finish on the rib and roller ends leaves asperities that penetrate the protective film created by the lubricant, thus creating higher friction. The large end faces on the rollers are slightly radiused to provide a desirable contact area, but when the crown becomes too pronounced, it serves to further concentrate the axial forces between the rollers and rib in an area that is too small. Pronounced runout in the large end face of a roller will destabilize the oil film between the end face and the thrust rib and contribute to higher torque. Typically, the roller end faces contact the thrust rib relatively far from the large end of the cone raceway, and this creates a relatively large moment arm between the point of contact and the raceway. The large moment arm demands a relatively high torque to rotate the roller about its axis, and this translates into torque required to rotate the cone of the bearing. Finally the typical bearing, owing to the crown on the rollers, has the center of contact between the rollers and the raceways generally located midway between the ends of the rollers. Thus, a relatively large moment arm exits between the contact at the large end of the roller and the center of contact along the raceways. The contact force acting along this moment arm tends to skew the rollers, and that in turn increases the torque in the bearing.
The present invention resides in a tapered roller bearing having races provided with opposed raceways, a thrust rib having a rib face at the large end of the raceway on one of the races, and tapered rollers between the races and having side faces along which they contact the raceways and large end faces along which they contact the rib face. The raceways, the rib face, the side and large end faces of the rollers, as well as the rollers themselves are all configured such that the bearing reduces churning of the hydrodynamic oil film within it to a minimum, demands little torque, and experiences minimal wear. The invention also resides in an automotive differential having a pinion supported on such a bearing.